1. Field of the Invention
The present invention relates to a hydraulic control apparatus for an automatic transmission, particularly a hydraulic control apparatus for an automatic transmission having the gear implemented by engagement of a friction engagement element.
2. Description of the Background Art
Conventionally known is an automatic transmission provided with a low reverse brake that engages in response to supply of oil pressure when the first speed gear or reverse gear is to be implemented. In such an automatic transmission, there is known the technique of preventing the low reverse brake from engaging to implement a reverse gear when the shift lever is erroneously shifted to the reverse position during forward driving.
Japanese Patent Laying-Open No. 05-322013 discloses an oil pressure controller including a reverse inhibit valve that inhibits implementation of a reverse gear. This oil pressure controller includes a line pressure control valve, a solenoid valve receiving the line pressure as the supply pressure to operate in response to a switch control signal to output a control oil pressure, and a reverse inhibit valve that inhibits implementation of a reverse gear under switching control by the control oil pressure output from the solenoid valve. When determination is made that implementation of a reverse gear is to be inhibited, the line pressure is reduced prior to output of a switching control signal to the solenoid valve. The switching control signal is output to the solenoid valve under such a state to execute switching of the solenoid valve.
The oil pressure controller disclosed in the aforementioned publication has the solenoid valve switched in response to occurrence of a state to inhibit implementation of the reverse gear, whereby switching of the reverse inhibit valve is controlled. Accordingly, the R-range pressure supplied from the manual valve to the brake and clutch to implement the reverse gear is cut off by the reverse inhibit valve. As a result, implementation of the reverse gear is inhibited. Prior to switching of the solenoid valve, the line pressure is reduced. The solenoid valve is switched under the reduced pressure. Accordingly, the load during the switching operation of the solenoid valve becomes smaller. Thus, a lighter and economic solenoid valve having lower oil pressure for the switching operation is available.
With respect to a reverse inhibit valve that suppresses implementation of a reverse gear, there is known the technique of feeding the oil pressure output from the reverse inhibit valve back thereto.
Japanese Patent Laying-Open No. 08-159275 discloses a reverse inhibit valve having the oil pressure output therefrom feed back. This reverse inhibit valve includes a spool that can selectively establish a forward gear position and a reverse gear position, a forward pressure oil chamber provided at one end of the spool, a reverse pressure feedback oil chamber provided at the other end of the spool, a governor pressure oil chamber provided between a small diameter land and a large diameter land, a governor pressure input port through which governor pressure is input, a governor pressure output port from which governor pressure is output,.a reverse pressure input port through which reverse pressure is input, a reverse pressure output port from which reverse pressure is output, and a spring in the reverse pressure feedback oil chamber. D-range pressure is supplied to the forward pressure oil chamber. Governor pressure is supplied to the governor pressure oil chamber. The reverse pressure output from the reverse pressure output port is fed back to the reverse pressure feedback oil chamber. The spring urges the spool in a direction against the D-range pressure of the forward pressure oil chamber and the governor pressure of the governor pressure oil chamber.
In accordance with the reverse inhibit valve disclosed in the aforementioned publication, the spool takes a reverse gear position by the urging force of the spring if the vehicle speed and the governor pressure are low when the reverse range (position) is selected through the select lever. At this stage, communication is established between the reverse pressure input port and the reverse pressure output port by the spool. Further, communication between the governor pressure input port and the governor pressure output port is cut. Accordingly, the R-range pressure is supplied to the hydraulic servo, whereby a reverse gear is implemented. When communication is once established between the reverse pressure input port and the reverse pressure output port, the reverse pressure output from the reverse pressure output port is fed back to the reverse pressure feedback oil chamber, whereby the spool is urged to the reverse gear position. Since the spool is held at the reverse gear position and communication between the governor pressure input port and governor pressure output port is cut off by the spool, the reverse gear can be maintained.
If a reverse inhibit valve that has the reverse pressure fed back, such as the reverse inhibit valve disclosed in Japanese Patent Laying-Open No. 08-159275, is applied to the oil pressure controller disclosed in Japanese Patent Laying-Open No. 05-322013, control of the reverse inhibit valve will be delayed, leading to the possibility of delay in the inhibition of the reverse gear. This problem will be described with reference to FIG. 6.
In the case where manual valve 5002 corresponds to a reverse position, R-range pressure from manual valve 5002 is supplied to a reverse inhibit valve 5004. At this stage, a solenoid 5006 is turned off when in the state to implement a reverse gear. Therefore, the spool of reverse inhibit valve 5004 takes the left-side state in FIG. 6 by the urging force of the spring provided in feedback oil chamber 5008. In this state, the R-range pressure supplied from manual valve 5002 is provided to the hydraulic servo of a brake 5010. Accordingly, brake 5010 engages, whereby a reverse gear is implemented. The oil pressure supplied to the hydraulic servo of brake 5010 is delivered back to feedback oil chamber 5008 as the feedback pressure.
In the case where implementation of the reverse gear is to be inhibited, solenoid 5006 is turned on, whereby the spool of reverse inhibit valve 5004 is switched to the right-side state in FIG. 6 by the oil pressure output from solenoid 5006. When the spool of reverse inhibit valve 5004 attains the right-side state in FIG. 6, the R-range pressure is cut off by reverse inhibit valve 5004 to prevent engagement of brake 5010. Accordingly, implementation of the reverse gear is inhibited.
If feedback pressure remains in feedback oil chamber 5008 during the switching of the spool of reverse inhibit valve 5004 to the right-side state in FIG. 6, the feedback pressure will oppose the oil pressure output from solenoid 5006. Therefore, control of. reverse inhibit valve 5004 will be delayed, leading to the possibility of delay in inhibition of the reverse gear.